BBC
Rear Admiral Michael Dumont expressed the opinion on behalf of the Joint Chiefs of Staff in a letter to Congressman Ted Lieu.
Mr Dumont said calculating "even the roughest" potential casualty figures would be extremely difficult.
He also gave some detail on what the first hours of a war would involve.
"The only way to 'locate and destroy - with complete certainty - all components of North Korea's nuclear weapons programs' is through a ground invasion," he wrote in response to
http://www.bbc.com/news/world-us-canada-41878123
